Manufacture of cascade image amplifiers



June 25, 1963 N. A. SLARK 3,095,251

MANUFACTURE OF CASCADE IMAGE AMPLIFIERS Filed Jan. 15, 1960 INVENTOR wmw WM ATTQRNEYS States Uite This invention relates to the manufacture of cascade image amplifiers, that is to say to image amplifiers Wherein an optical image to be amplified falls upon a photocathode the electrons from which are accelerated and focussed upon a fluorescent screen to form thereon a stronger optical image the light from which produces photo-emission from another photo-cathode which in turn emits electrons which are accelerated and focussed upon a further fluorescent screen where a still stronger optical image is formed to produce light to activate still another photo-cathode and so on through a desired cascade of image amplifying stages, each consisting of a photo-cathode and associated fluorescent screen, until an image of desired intensity is obtained on a final fluorescent screen.

FIG. 1 of the accompanying drawings shows schematically a typical known cascade image amplifier of the so-called sandwich type. Deposited on the inside of one transparent end wall A of the envelope B of the tube is a photo-electric cathode C on which an optical image of a subject D (represented as an arrow) is focussed by means of a suitable lens system represented by a lens E. Electrons emitted from the cathode C are accelerated and focussed by uniform electric and magnetic fields (generated by means not shown) on a fluorescent layer F on a thin transparent membrane G extending across the tube envelope. On the other side of the membrane G is a photo-cathode H. There are several further such mem-' branes G, G" and so on, spaced along the tube, each with a fluorescent layer F, F on one side and a photo-cathode H, H" on the other. On the inside of the window I at the other end of the tube envelope is a final fluorescent layer K. The applied electrical and magnetic fields are such as to cause the photo-electrons emitted from the cathode C to produce a stronger reproduction of the original optical image on the fluorescent screen F, the light from which causes photo-emission from the cathode H to occur and produce a still stronger optical image on the fluorescent screen P which in turn causes photo-emission from the cathode H to produce a still stronger image on the fluorescent screen F and so on until a bright image is produced on the final screen K.

Great diificulties are experienced in producing a really satisfactory tube as exemplified in FIGURE 1. One of the diflicul-ties lies in the fact that the manufacturing processing of the cathodes involves heat treatment. If all cathodes are heat treated together it is almost if not quite impossible to subject them to the same rates of heating and cooling and nonuniformity results from this fact, some of the cathodes turning out to be inferior to others. If, however, the cathodes are heat treated separately, one after the other, when in the envelope, it is extremely dificult to prevent the heat treatment applied to one cathode from adversely affecting and even seriously damaging those which have already been treated. Moreover, caesium is introduced during the heat treatment of the cathodes and may attack and damage the fluorescent deposits and/or become deposited on the inside of the envelope Walls and cause so-called dark current effects when the finished tube is put into operation. Moreover,

" atent 3,fl95,25l Patented June 25, 1963 the necessity of minimising damage to fluorescent screens by caesium and the necessity, in tubes in which the cathodes are processed individually, of minimising risk of damage to an already processed cathode when processing another cathode, involve that the successive membranes shall be spaced relatively far apart and this in turn involves expensive heavy and carefully adjusted magnetic field systems to ensure accurate focusing of the photoelectric emission from each cathode on the next fluorescent screen in the cascade.

The present invention seeks to overcome the foregoing defects and to provide methods of manufacture which will result in tubes having cathodes of practically uniformly good quality, and which can be made relatively small and compact, with only small spacing between the successive membranes, and with only small dark current. Indeed, it may be possible to employ the present invention to produce satisfactory tubes with the membranes so close togetherspaced by only a few millimetresthat eflective focusing can be obtained without any applied magnetic focusing fields at all.

According to this invention a method of manufacturing a cascade image amplifier includes the steps of mounting a plurality of separate membranes on a carrier member in an evacuated space, mounting parts of the envelope while still in said evacuated space, bringing the separated parts of the envelope together and sealing them to one another to produce a hermetically sealed closed envelope with the processed cathodes in position inside it, and

releasing the vacuum and removing the sealed envelope from the space.

Preferably the membranes are processed one at a time, While still on the carrier member, by being brought in turn into a heat treating oven mounted in the evacuated space.

Preferably the carrier member is a movable member adapted to carry the membrane in such positions thereon that, on moving said member, said membranes are moved through a path which passes through the heat treating oven and through a space adjacent one of the separated par-ts of the envelope, said membranes being lightly held in said positions so that, when a membrane has been brought into said space adjacent one of the separated parts of the envelope, it can be readily transferred from its position in said carrier member to a position in said one envelope part.

Preferably said movable carrier member is also adapted to carry the other separated part of the envelope,.said other pant of the envelope also being moved through a path which passes through the heat treating oven and through said space adjacent said one part of the envelope, said other part of the envelope being so held in position that it may be readily transferred from its position in said carrier member to a position adjacent and in contact with said one part of the envelope.

Preferably the image amplifier comprises transparent membranes each with a cathode on one side and a fluorescent layer on the other, and these membranes are mounted each across a short tubular mount which is lightly held in position on the carrier member and is adapted to fit into a I tubular member constituting one of the envelope parts.

According to a feature of the invention an apparatus for carrying out a method of manufacture as hereinbefore set forth comprises an evacuatable chamber with a removable lid, a wheel mounted in said chamber and constituting a carrier member adapted lightly to hold a plurality of rotation of the wheel, said membranes and said closure portion carried thereby will pass in succession through said oven, means for rotating said'wheel fromoutside said chamber, means in said chamber for holding/a main envelope'portionwith'a space adjacent one end thereof said space being :so positioned that, on rotation of the Wheel, said membranes and said closure portion carried thereby will pass in succession through said space, a bellows structure for permitting the main envelope portions to be reciprocatedfrom outside the chamber to close and open said space and thereby transfer said membranes in turn and finally saidclosureiportion in said space to the inter-ior of the main envelope portion, and means permitting said envelope portions to be pressed tightly together to cold seal together metal seal members on the adjacent faces ofthe envelope portions.

FIG. Z'of the accompanying drawings illustrates one way of carrying out the invention and shows in schematic section a form of apparatus for practising: the method of the invention, FIG. 3 shows, so far as is necessary, a preferred modification of the apparatus of FIG. 2.

Referring to .FIG. 2 there is an evacuatable chamber comprising a helmet-like lid member 1 of stainless steel and a base plate 2 having a hole 3 through which the chamber canbe pumped out via a pipe (not shown) fitted vacuum-tight tothe hole. Thelid member 1 is bolted to the base 2 with interposed packing pieces 4to provide avacuum-tight joint. Thelid member has a number of inspection windows in the top and sides one of these windows being shown at-S.

In the upper part of the chamber is a tubular envelope carrier 6 in which can be fitted the, major portion 7 of the envelope of an image amplifier. The part 6 is longitudinally movable between guides 8 and, is carried by a bellows structure 9 as shown to permit itto be moved longitudinally by meanssuch as a hand wheel 10' and screwed rod 11 from outside the chamber. .The envelope portion 7 is formed with an end copper or other suitable flange 7'.

13 suitably mounted in the chamber. The envelope portions 7 and 12 are spaced apart opposite one another in suchv a manner that by moving the part '6 endwise the flanges 7" and 12 can be brought into contact, said flanges being made of such material that they cold weld to one of any suitable type adapted to provide satisfactory heat treatment for processing a photo-cathode in accordance with known processing practice. 7 Within this oven are provided the necessaryevaporators or other -means;required to. deposit the required photo-cathode materials.

Membranes on which photo-cathodes are to be formed are carried on a rotatable conical wheel '15-which is rotatable on an axial shaft 16. The wheel can be rotated from outside the chamber in any convenient manner, for

example, through a magnetic drive operable from outside the chamber. So as not to complicate the drawing, the means for rotating the wheel 15 are not shown.

The wheel carries a ring of membranes on which photocathodes are to be formed. Each photo-cathode is formed on one side of a transparent membrane :17 on the other, side of which is a fluorescent deposit except for the membrane on which the first photo-cathode is to be'formed, this membrane, :of course, not having-a fluorescent screen. Each membrane is mounted across a short tubular glass mount '18 which-is lightly held to the wheel by a spring holder (not shown). The mounts #18 are of suitable size The completeenvelo'pe consists of the portion '7 and a closure portion 12 which is provided with'anend copper or similar flange 12 and is fitted into a fixed part 1 4. to fit into the envelope portion 7. The oven 14*, the wheel 15, the holders for the mounts 18, and the parts holding the envelope portions 7 and 12, are in such mutual relationship that by rotating'the wheel 15, the mounts 18 will traverse a circular path which passes through the oven 14 and the space between'the envelope portions 7 and 12 when those portions are in the positions shown in' FIGURE 2.

The apparatus is used'as follows: The lid' is taken off the base 2. A number of mounts 18'each with its membrane having'a fluorescent screen on one side are' placed in the spring'holders on the wheel 15; an envelope portion 7 is put in position in the member 6 and an envelope closure portion 12 with an already prepared fluorescent screen on'the inside of its window, is placed in the" part 13. The lid 1 is then put into position on the base 2 and the chamber is pumped out and maintained in an evacuated condition. The wheel 15 is rotated through its magnetic drive to bring the'membranes carried byit in turn and one at a time into the oven 14 where each is heat treated and processed in accordance withiknown'practice to form a photo-cathode. When the photo-cathodes have been formed they can be examined for condition by.in-

.spection through one or' other-of the inspection windows.

in the chamber without removing them from the said chamber. The numberof' membranes carried by the wheel 15 is greater than the number of cathodes required in the complete-tube sothat the ones having the best photocathodes can be selected for putting in the tube and any bad ones rejected. One of'the selected cathodes is then brought into position between the envelope portions 7 and 12' by rotating the wheel. operated to cause the'part 6, with the envelope portion 7:

insideit, to slide over the mount 18 which is in the space.

between the portions 7 and 12. The part 6 is then with drawn again by means of the hand wheel 19. During this withdrawal friction between the inside of the portion 7 and the mount 18-is sufiicient to pull the said mount from its spring holder. This mount will now be 'inside' portion 7. Another mount 18 isnow brought into position-between the envelope portions 7 and 12, the part 6 is again reciprocated and this further mount is transferred into the envelope portion 7.. This transference of selected photo-cathodes on the mounts 18 from the wheel to the envelope portion 7 continues until the desired number of photo-cathodes and fluorescent screens is within the said envelope portion 7. It'will be seen that the spacing of the membranes 17 in the envelope portion 7 will be determined by the lengths of the'cylinders 18.

Thewheel and screw mechanism 10-11 is now removed leaving thechamber still sealed. A portable by draulic press (not shown) islowered over 'the'chamber. and pressure is applied between the parts 19 and 24} which are formedto constitute thrust blocks for taking the pressure of the hydraulic press. Also the wheel 15 is rotate-d'until a cut-out portion inthe rimthereof is brought between the envelope portions 7 and 12. These two portions are now brought into contact (the cut-out portion of the wheel 15 permitting this) by the hydraulic press and suflicient pressure is applied between them to cause a cold weld tooccur between the copper flanges 7" and V 12'. The envelope is thus sealed. Vacuum is then released from the chamber, the lid 1 taken off the base 2 and the sealed envelope removed.

In order to obtain a good vacuum in the envelope, it

is usually advisable to getter it in accordance with,

In such cases a getter may be conven-- The hand wheel '10' is then spirals so as to provide a uniform potential gradient between one membrane and the next when the image amplifier is in use. Of course, external connectors (not shown) to the membranes and passing through the envelope portion 7 may be provided as required.

In the case of image amplifiers of high gain there is a possibility that X-rays caused by electrons striking the final fluorescent screen of the tube may be absorbed in the first photo-cathode and produce dark current photoelectrons and, in extreme cases, this effect may be regenerative and result in loss of picture. Trouble from X-rays may be greatly reduced and in practice eliminated by making the membranes of lead glass.

An advantage of the invention is that as a result of the individual treatment of individual photo-cathodes without affecting other photo-cathodes and without difficulties due to liberated caesium, it is possible to make a high gain image amplifier with the successive photo-cathodes spaced apart by much smaller amounts than are usually regarded as necessary. Indeed, it may be possible to adopt spacings as low as only a few millimetres each. While this obviously reduces the size of the amplifier, it also has the greater advantage that it enables focusing on the successive fluorescent screens to be obtained without the use of externally applied magnetic fields and the consequent elimination of solenoids or magnets for such fields not only reduces cost, but greatly reduces weight-an important matter for airborne apparatus in particular.

In some cases it is desired that the first cathode of a cascade image amplifier tube shall be deposited direct on one of the end walls of the envelope and the final fluorescent screen deposited direct on the other end wall of said envelope. Such a tube is represented in FIGURE 1 and the present invention can be used in the manufacture of such tube. FIGURE 3 shows, so far as is necessary for an understanding thereof, a modification of the apparatus of FIGURE 2 which permits this method of manufacture to be done. Where such a tube is to be made the portion 7 of the envelope (FIGURE 3) is provided with a preformed fluorescent screen 21 deposited on the inside of its end wall and the end closure portion 22 of the envelope is mounted in a holder on the wheel 15. The spacer 22 does not carry a membrane and is placed in the tube 7 prior to its insertion in the tube 6. After a photocathode has been formed on the inside of the end wall of said portion 22 while it is on the wheel and after the selected membrane mounts have been put into the envelope portion 7, the wheel is turned to bring the portion 22 opposite the portion 7. The part 6 is again reciprocated and the closure portion 22 is transferred from its spring mount into the portion 7 where it is held by friction and the tube is sealed as already described. Obviously, when a tube is to be assembled in this way the membrane mounts must be placed in their holders on the wheel the other way round so that, in the finished tube, each photo-cathode faces a fluorescent screen. Preferably the wheel 15 carries more than one closure portion 22 and that one having the best photo-cathode is selected for use.

In preferred practice in carrying out the invention, there is provided an external oven (not shown in the drawings) which is big enough to contain the whole evacuatable chamber of FIGURE 2 and which is raised to a temperature of several hundred degrees during the initial evacuation. This facilitates the obtaining of a high degree of cleanliness and enables a really high vacuum to be attained during processing. All the materials used in this case must be such as to Withstand this temperature and such as will outgas properly. It is also of advantage to provide getters in the evacuatable chamber and to fire them immediately before cathode processing is commenced, thereby removing residual gas traces which might otherwise harm the cathodes. traps in the pipe line from the evacuatable chamber to the vacuum pump, the upper of which can be baked at the same temperature as the chamber so as to aid in securing a high vacuum during cathode processing.

I claim:

1. A method of manufacturing a cascade image amplifier including the steps of mounting a plurality of separate membranes on a carrier member in an evacuated space, mounting parts of the envelope of said amplifier in separated positions in said space, said parts being separated from said carrier member, depositing photo-cathode material on and heat treating said membranes to form photoelectric cathodes thereon while mounted on said carrier member in said space separated from said envelope parts, transferring desired processed cathodes from said carrier to predetermined positions in one of said envelope parts while still in said evacuated space, bringing the separated parts of the envelope together and cold sealing them to one another to produce a hermetically sealed closed envelope with the processed cathodes in position inside it, and releasing the vacuum and removing the sealed envelope from the space.

2. A method according to claim 1, wherein the step of heat treating said membranes is accomplished on one membrane at a time while the membranes are on the carrier member by bringing the membranes in turn into a heat treating oven mounted in the evacuated space.

3. A method according to claim 1, wherein the step of heat treating the membranes is accomplished by moving the membranes one at a time through an oven and wherein the step of transferring the cathodes to the envelope is accomplished by moving the membranes through a space adjacent one of the separated parts of the envelope lightly holding the membranes on said carrier and transferring said membranes one at a time from the carrier member to a position in said one part of the envelope.

4. The method according to claim 3, wherein the step of bringing together the separated parts of the envelope is accomplished by carrying the other separated part of the envelope on the carrier member through the oven into engagement with said one part of the envelope.

5. The method according to claim 4, including the step of mounting each membrane across a short tubular mount before the membranes are mounted on the carrier member and wherein the step of mounting the membranes on the carrier member is accomplished by engaging the tubular mounts with a cooperating tubular protrusion on the carrier member.

References Cited in the file of this patent UNITED STATES PATENTS 2,575,756 Fulton et a1 Nov. 20, 1951 2,713,532 McCullough July 19, 1955 2,792,271 Beggs May 14, 1957 2,837,396 Warren June 3, 1958 Provision is also preferably made for cold 

1. A METHOD OF MANUFACTURING A CASCADE IMAGE AMPLIFIER INCLUDING THE STEPS OF MOUNTING A PLURALITY OF SEPARATE MEMBRANES ON A CARRIER MEMBER IN AN EVACUATED SPACE, MOUNTING PARTS OF THE ENVELOPE OF SAID AMPLIFIER IN SEPARATED POSITIONS IN SAID SPACE, SAID PARTS BEING SEPARATED FROM SAID CARRIER MEMBER, DEPOSITING PHOTO-CATHODE MATERIAL ON AND HEAT TREATING SAID MEMBRANES TO FORM PHOTOELECTRIC CATHODES THEREON WHILE MOUNTED ON SAID CARRIER MEMBER IN SAID SPACE SEPARATED FROM SAID ENVELOPE PARTS, TRANSFERRING DESIRED PROCESSED CATHODES FROM SAID CARRIER TO PREDETERMINED POSITIONS IN ONE OF SAID ENVELOPE PARTS 